This invention pertains to optical viewers for microfiche.
Microfiche viewers of single frames are known to the art. Except for making a hard copy from microfiche, such a viewer is required to present a transient optical image at substantially the size of the original document to one who wishes to read the information involved.
In simple terms, microfiche viewers may be considered as a "magic lantern" slide projector of antiquity with the addition of a built-in screen.
However, today sophisticated microfiche film-positioning means are required to successively present single frames from the microfiche. An over-view of the whole fiche is not desired.
The art has made provision for infrequently altering the magnification of viewers. This has been for the purpose of properly reproducing fiche that were originally optically reduced by specific ratios, such as 24 or 48. Alteration has usually required removing one objective lens and replacing it with another of different focal length.